In recent years, integrated circuit devices have steadily become smaller, with increased internal signal speeds. As a result, their power densities have become greater, causing increased heat generation during operation. Overheating of integrated circuit devices may result in their failure or destruction. The need for effective heat removal techniques in this area is a basic problem.
The problem of heat removal from integrated circuit devices is accentuated when they are mounted on a supporting substrate with solder terminals for connecting electrodes on the devices to electrically conductive traces on the substrate. In such a solder bonded device a significant portion of the heat is usually removed from the side of the device in contact with the substrate. The removed heat travels through the substrate, to a heat exchanger in thermal contact with the substrate. A dense placement of a plurality of such devices on a single multi-layer, or "thick film", ceramic substrate further significantly increases the amount of heat to be removed through the substrate.
In prior art systems the above mentioned heat exchanger is implemented as a heatsink placed in thermal contact with a surface of the substrate on which no circuit devices are mounted. A gaseous or liquid coolant may further be used to remove the heat from the heatsink. A key consideration in maximizing heat flow away from the integrated circuit devices, is maximizing thermal conductivity between the substrate and the heatsink. Air pockets between the substrate and the heatsink increase thermal resistance, and are therefore undesirable.
Substrate warpage induced during fabrication accentuates this problem. Substrates are typically a sandwich construction, with alternating layers of copper and ceramic. Substrate fabrication involves a repetitive firing process. Because the copper and ceramic substrate layers have dissimilar thermal expansion coefficients, the repetitive firing process causes the substrate to be initially warped. The degree of the initial warpage is not accurately predictable. If a warped substrate were simply mated with a flat heatsink surface, unacceptably large air pockets would exist between the substrate and the heatsink.
In addition to these relatively large air pockets, smaller air pockets may be caused by microscopic irregularities in the finishes of the substrate and heatsink attachment surfaces. To reduce both large and microscopic air pockets between substrate and heatsink, and thereby provide a more thermally conductive interface, there currently exist several approaches.
One approach is to use a thermally-conductive adhesive between the heatsink and the substrate. The adhesive compensates for the initial warpage of the substrate, and will conform to any surface irregularities as well. As a result, essentially all air pockets between substrate and heatsink are eliminated. However, there are disadvantages. First, even so-called "flexible" adhesives form a strong bond between the substrate and the heatsink. The rigidity of the adhesive bond increases the risk of failure due to fatigue caused by cyclical stress. Cyclical stress occurs as the device temperature changes through being powered u and down. During each of these power cycles, the adhesive bond transmits stress resulting from the dissimilar thermal expansion rates of the substrate and heatsink to the substrate. This stress, and the resultant fatigue, may cause the substrate to crack or break, causing failure in the electrical circuit carried thereon. The adhesive bond also prevents separation of the parts for repair purposes. Furthermore, adhesives generally are relatively poor heat conductors and contribute to the overall thermal resistance between the substrate and the heatsink. Finally, applying adhesives in a manufacturing environment requires expensive and cumbersome dispensing and curing equipment, making their use undesirable from a manufacturability perspective.
A second approach i to introduce a gasket-type interface material between the two parts. A separate gasket part is placed between substrate and heatsink to act as a thermal bridge. Gaskets are relatively easy to handle and install at assembly time. Unlike adhesives, they do not form a rigid bond between the substrate and heat sink. Typical gasket materials, however, lack the compliance necessary to compensate for the initial warpage of ceramic substrates, and their thermal conductivity is relatively poor. Gaskets also are generally not effective in reducing the microscopic air pockets resulting from irregularities in the surface finish of the parts.
A third approach uses a thermal grease to interface between the parts. Like thermal adhesives, thermal greases can essentially eliminate all air pockets between the parts. Unlike thermal adhesives, they produce a stress-free interface between the parts. The disadvantages of thermal greases are that they require cumbersome dispensing equipment at assembly time, have relatively poor thermal conductivity, and tend to migrate away from the interface. As the grease migrates away from the interface, air pockets may again result, increasing thermal resistance between the parts. Should the migrating grease come to rest on an electrical connection, it could interfere with the connection.
A new heatsink is desirable which will maximize thermal conductivity between the heatsink and the substrate by minimizing the air pockets between the two, and also be inexpensive to produce, and be easily repaired.